1. Field of the Invention
The present invention relates to process control systems arranged to accept sensing signals from the field, to process data represented by said sensing signals, and to control field process devices according to the processed data.
2. Description of the Prior Art
Known process control systems generally are connected to a variety of field devices issuing or requiring many different forms of analog signals, such as currents, voltages, resistances, etc. The data processing portion of a process control system, however, requires standardized signals. This necessitates a mechanism for signal conversion between the varied field signals and the standardized control signals.
In known prior art process control systems, signal conversion mechanisms are designed to suit the particular control system, either digital or analog, that is used, with no interchangeability. As a result, no generalized process control system has been available.
In one known type of process control system using electronic digital computer processing, the interface between digital processing signals and analog field signals samples multipoint analog signals in sequence with a multiplexer in response to commands from the computer, and then converts the signals to digital form and supplies them to the computer. Digital output signals from the computer are first converted to analog form and then supplied by a demultiplexer to multipoint controlled elements. In such arrangements, the multiplexer and demultiplexer must match the computer efficiency and require digital control circuits which complicate design, production, and servicing.
In another known arrangement, varied field analog signals are standardized, in analog form, before analog-to-digital conversion and after digital-to-analog conversion. In such devices, however, the standardized intermediate signal is a voltage or current signal varying in amplitude in a predetermined range. Such signals are vulnerable to noise and require intricate noise removing arrangements including a filter to remove normal noise, and isolation means to remove common mode noise. Practical circuits require a significant number of isolation means, necessitating considerable design complication.
In some known process control systems, a digital computer processor has an analog monitoring means and backup means. The field devices are connected to the monitoring and backup means through the multiplexer and demultiplexer, and must be isolated from the monitoring and backup means as well. The extra isolation means and interconnections add much complexity to the system.
Finally, known process control systems have signal transmission arrangements that are not entirely satisfactory. Signals commonly are transmitted over individual transmission lines, which are grouped into multicore cables handling proximate field sensing and control devices. The same cable, therefore, carries signals with a variety of forms, such as voltage, current, resistance, etc. As the size of a process increases, the number of cables and transmission lines also increases. The arrangement of input and output terminals at the control system, however, is dictated by the control system. To lead the signal transmission lines, wrapped in cables, to the individual terminals, transmission lines must be re-edited and regrouped with a significant expenditure of labor. Adding to the difficulty of re-editing is the fact that practical transmission lines are designed to withstand harsh environments and are thick in diameter.
Because of the foregoing problems, known process control systems have not been entirely satisfactory in interfacing varied field analog sensing and control signals with digital data processors.